Iridium Complexes of the Conformationally Rigid IBioxMe4 Ligand

Article pubs.acs.org/Organometallics

Iridium Complexes of the Conformationally Rigid IBioxMe4 Ligand: Hydride Complexes and Dehydrogenation of Cyclooctene Simone A. Hauser,† Ralf Tonner,‡ and Adrian B. Chaplin*,† †

Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, United Kingdom Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Straße 4, D-35032 Marburg, Germany

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Downloaded by FLORIDA ATLANTIC UNIV on September 6, 2015 | http://pubs.acs.org Publication Date (Web): September 2, 2015 | doi: 10.1021/acs.organomet.5b00658

S Supporting Information *

ABSTRACT: A method for accessing the formally 14 VE iridium(III) hydride fragment {Ir(IBioxMe4)2(H)2}+ (2), containing the conformationally rigid NHC ligand IBioxMe4, is reported. Hydrogenation of trans-[Ir(IBioxMe4)2(COE)Cl] (1) in the presence of excess Na[BArF4] leads to the formation of dimeric [{Ir(IBioxMe4)2(H)2}2Cl][BArF4] (3), which is structurally fluxional in solution and acts as a reservoir of monomeric 2 in the presence of excess halogen ion abstractor. Stable dihydride complexes trans-[Ir(IBioxMe4)2(2,2′bipyridine)(H)2][BArF4] (4) and [Ir(IBioxMe4)3(H)2][BArF4] (5) were subsequently isolated through in situ trapping of 2 using 2,2′bipyridine and IBioxMe4, respectively, and fully characterized. Using mixtures of 3 and Na[BArF4] as a latent source of 2, the reactive monomeric fragment’s reactivity was explored with excess ethylene and cyclooctene, and trans-[Ir(IBioxMe4)2(C2H4)2][BArF4] (6) and cis-[Ir(IBioxMe4)2(COD)][BArF4] (7) were isolated, respectively, through sacrificial hydrogenation of the alkenes. Complex 6 is notable for the adoption of a very unusual orthogonal arrangement of the trans-ethylene ligands in the solid state, which has been analyzed computationally using energy and charge decomposition (EDA-NOCV). The formation of 7 via transfer dehydrogenation of COE highlights the ability to partner IBioxMe4 with reactive metal centers capable of C−H bond activation, without intramolecular activation. Reaction of 7 with CO slowly formed trans-[Ir(IBioxMe4)2(CO)2][BArF4] (8), but the equivalent reaction with bis-ethylene 6 was an order of magnitude faster, quantifying the strong coordination of COD in 7.

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INTRODUCTION

of investigating low-coordinate NHC complexes of rhodium and iridium, we have recently begun to expand the coordination chemistry of IBiox ligands, seeking to exploit their conformational rigidity to avoid intramolecular cyclometalation reactions that can occur via C−H bond activation of the downward pointing alkyl and aryl NHC appendages.2,3 In particular, we have focused our efforts on IBioxMe4, which shares many structural similarities with the commonly employed ItBu ligand that has been shown to undergo cyclometalation reactions when partnered with reactive late transition metal fragments (Scheme 1).4,5 Our investigations to date have supported the hypothesized reactivity attenuation, contrasting facile cyclometalation observed in reactions of ItBu with dimeric rhodium and iridium precursors.5 In the case of rhodium, formally 14 VE Rh(I) complexes have been isolated and fully characterized in solution and the solid-state: [Rh(IBioxMe4)2(COE)]+ (A; COE = cyclooctene) and [Rh(IBioxMe4)3]+ (B; Scheme 2).6 The rigid geometry of IBioxMe4 appears to prohibit the adoption of any significant agostic interactions, and remarkably, despite the high degree of electronic unsaturation, the homoleptic complex

Bioxazoline-derived imidazolylidene ligands (IBiox) developed by Glorius and co-workers are N-heterocyclic carbenes (NHCs) with considerable potential for organometallic chemistry and catalysis (Chart 1)the readily adapted bioxazoline backbone provides a versatile scaffold for the synthesis of achiral and chiral ligands with tunable steric bulk.1 However, despite finding notable application in palladiumcatalyzed cross-coupling reactions, the coordination chemistry of IBiox ligands has remained largely unexplored. With the aim Chart 1. IBiox Ligands Developed by Glorius

Received: July 31, 2015

© XXXX American Chemical Society

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DOI: 10.1021/acs.organomet.5b00658 Organometallics XXXX, XXX, XXX−XXX

Organometallics

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Scheme 1. Hypothesized Reactivity Attenuation

RESULTS AND DISCUSSION

With the dihydride bis(NHC) complex [Ir(ItBu)2(H)2]+ (E) as a structural precedent,5b the preparation and isolation of the direct IBioxMe4 analogue 2 was targeted (Scheme 3). Complex E is formally 14 VE Ir(III) and stabilized in the solid-state by the formation of strong agostic interactions from the tert-butyl substituents of the NHC: interactions characterized by short Ir···HC contacts of 2.653(10) Å. Given the apparent stability of IBioxMe4 to intramolecular activation, the synthetic route associated with E involving hydrogenation of bis-cyclometalated [Ir(ItBu′)2]+ is not practical. Instead, the preparation of 2 was attempted through the reaction of isolated 1 with dihydrogen (1 atm) following halide extraction using excess (2.0 equiv) Na[BArF4] (ArF = 3,5-C6H3(CF3)2) in 1,2difluorobenzene solution (293 K). Analysis of the reaction mixture using 1H NMR spectroscopy after 5 min and placement under an argon atmosphere showed complete conversion of the starting material into a new hydride species, characterized by broad singlet resonances at δ 4.27, 1.62, and −35.85 in a 4:12:1 ratio, alongside cyclooctane (COA, Scheme 3). Repeating the reaction instead with 0.5 equiv of Na[BArF4] resulted in the formation of a compound with the same spectroscopic characteristics, suggesting that the chloro-bridged Ir(III) dimer [{Ir(IBioxMe4)2(H)2}2Cl][BArF4] (3) is formed in both cases, rather than monomeric 2. Indeed, this dimeric complex was subsequently isolated in 77% yield under the latter conditions and fully characterized (vide inf ra, Figure 1). The formation of 3 implicates the intermediate presence of 2 in solution and either competitive binding to {Ir(IBioxMe4)2(H)2Cl} or partial solubility of NaCl in fluoroarene solution. With the IBioxMe4 ligand less predisposed to the formation of stabilizing agostic interactions than ItBu, both suggestions are in accord with higher expected reactivity of 2 relative to Nolan’s dihydride complex E. Invariant 1H NMR spectra of 3 were recorded after 24 h at ambient temperature in CD2Cl2 and 1,2-C6H4F2, implying good solution stability; the 19F{1H} NMR spectrum in CD2Cl2 displayed only a signal attributed to the [BArF4]− anion. Consistent with retention of the dimeric formulation in solution, the broad ligand resonances observed at δ 4.42 (16H) and 1.64 (48H) in the 1H NMR spectrum at 298 K decoalesced on cooling to 250 K (CD2Cl2, 500 MHz), resolving diastereotopic methylene (δ 4.31, 4.47; 2JHH = 8.4 Hz) and two different methyl signals (δ 1.48, 1.69). The 4H hydride resonance also sharpened significantly on cooling from 298 to 250 K (fwhm = 43 Hz vs 6 Hz). At both temperatures the measured T1 times are consistent with hydride ligands (375 ms at 298 K; 276 ms at 250 K).11 Further cooling to 200 K resulted in the onset of broadening, implying a loss of both possible symmetry planes of the ligand should occur in the slow exchange regime; the hydride ligand remained sharp (fwhm = 6

Scheme 2. Low Coordinate Rhodium and Iridium IBioxMe4 Complexes (L = IBioxMe4)a


Article

a M = Rh, Arf = 2,3-C6H3F2; Ir, Arf = 2-C6H4F, 2,3-C6H3F2, 2,4,6C6H2F3.

displays complete solution stability (CD2Cl2 or 1,2-difluorobenzene, 48 h at 293 K). Attempts to prepare analogous iridium complexes lead instead to intermolecular C−H bond activation of the fluoroarene solvents employed (i.e. C, D in Scheme 2).7 Such reactivity is fully inline with the more energetically accessible higher oxidation states of the heavier group 9 congener and reinforces the ability to partner IBioxMe4 with reactive metal centers without intramolecular activation.8 Cyclometalation, double cyclometalation, and dehydrogenation of the N-alkyl and aryl substituents of NHC ligands are otherwise well documented for iridium systems.5b,c,9 The 4-coordinate Ir(III) complex C was implicated in the preparation of D (Arf = 2,3-C6H3F2) from the well-defined Ir(I) precursor 1. While we were not able to directly isolate C, its formulation was substantiated in situ using 1H and 19F NMR spectroscopy and through reaction with 2,2′-bipyridine, which afforded a more readily handled, coordinately saturated product. With these results in mind and intrigued by the possibility of accessing reactive iridium bis(NHC) synthons, we now report our efforts at preparing the iridium hydride species {Ir(IBioxMe4)2(H)2}+ (2) and its subsequent reactivity, principally with alkenes. Compounded by potentially facile reductive elimination of dihydrogen as a thermodynamic driving force,10 low-coordinate iridium hydride species such as these represent good targets to test the robustness of IBiox ligands.

Scheme 3. In Situ Preparation and Derivatives of Dihydride Complex 2 (L = IBioxMe4)a

a

[BArF4]− anions omitted for clarity. All reactions in 1,2-C6H4F2 at 293 K. B


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Organometallics

Figure 1. Solid-state structures of iridium IBioxMe4 hydride complexes. Thermal ellipsoids for selected atoms are drawn at the 50% probability level; most hydrogen atoms (hydrides located in 4), anions, and solvent (5) are omitted for clarity; only one of the two independent molecules is shown for 5. Selected bond lengths (Å) and angles (deg), 3: Ir1−Cl2, 2.4499(13); Ir1A-Cl2, 2.4817(13); Ir1−C3, 2.047(4); Ir1−Cl8, 2.038(4); Ir1A-C3A, 2.038(4); Ir1A-C18A, 2.034(5); Ir1−Cl2−Ir1A, 171.47(7); C3−Ir1−C18, 176.35(18); C3A−Ir1A−C18A, 178.62(19); all ΘNHC > 175. 4: Ir1−H2 = Ir1−H3, 1.36(2); Ir1−C4, 2.064(3); Ir1−C19, 2.053(3); Ir1−N34, 2.175(2); Ir1−N45, 2.199(2); C4−Ir1−C19, 161.10(10); ΘNHC(@C4), 170.1(3); ΘNHC(@C19), 174.4(3). 5: Ir1−C2, 2.043(2); Ir1−C17, 2.050(3); Ir1−C32, 2.124(3); C2−Ir1−C17, 166.70(10); C2−Ir1−C32, 97.55(9); C17−Ir1−C32, 95.55(10); all ΘNHC(@C2,C17,C32) > 175; Ir1A−C2A, 2.059(2); Ir1A−C17A, 2.051(3); Ir1A−C32A, 2.115(3); C2A− Ir1A−C17A, 163.39(10); C2A−Ir1A−C32A, 98.31(9); C17A−Ir1A−C32A, 98.16(10); ΘNHC(@C2A), 173.8(2); ΘNHC(@C17A), 173.3(3); ΘNHC(@C32A), 179.3(3).14

respective aryl fluoride analogues trans-[Ir(IBioxMe4)2(2,2′bipyridine)(C 6 H3 F 2 )(H)]+ and D.7 Verification of the structures in solution was readily achieved through the presence of low frequency hydride resonances at δ −21.33 (T1 = 701 ms) and −28.57 (T1 = 940 ms), respectively, in CD2Cl2 (298 K, 500 MHz). To further explore the reactivity of the {Ir(IBioxMe4)2(H)2}+ fragment 2 in situ, mixtures of 3 and Na[BArF4] were reacted with ethylene (1 atm) and COE (10 equiv)Scheme 4. The

Hz; T1 = 446 ms). To account for these time-averaged spectra, a dissociative mechanism is suggested, involving fragmentation of 3 into {Ir(IBioxMe4)2(H)2Cl} and 2 followed by rotation of the IBioxMe4 ligands about the Ir−CNCN bonds. Inspection of the solid-state structure of 3 indicates that an alternative suggestion involving such a rotation in the intact dimer is unlikely due to the close proximity of the NHC ligands to each other (Figure 1). The bridged structure of 3 in the solid state is rather unusual, with an offset (|C3/18−Ir1−IrA−C3A/18A| = 50.3(2)/ 47.7(2)°) face-to-face alignment of the coordinated NHC ligands, almost linear bridging chloride ligand (Ir1−Cl2−Ir1A = 171.47(7)°), and unequal Ir−Cl bonds (2.4499(13) vs 2.4817(13) Å). The closest iridium NHC precedent featuring a single bridging chloride ligand [{Ir(COD)(IMes)}2Cl]+ (COD = 1,5-cyclooctadiene) is in contrast characterized by a bent bridging chloride ligand (Ir−Cl−Ir = 126.91(8)°) and more symmetrical Ir−Cl distances (2.407(2)/2.418(2) Å);12 however, near linear arrangements have been observed in iridium(III) complexes bearing chelating or monodentate phosphine ligands.13 Despite the dimeric resting state, mixtures of 3 and Na[BArF4] act as a latent source of the reactive monomeric fragment 2 in 1,2-difluorobenzene. As evidence, reaction of isolated 3 and 2,2′-bipyridine in the presence of 1.1 equiv of Na[BArF4] resulted in quantitative formation of the 6coordinate dihydride complex 4 (Scheme 3). Rather than using the isolated dimeric product, mixtures of 3 and Na[BArF4] generated in situ from hydrogenation of 1 in the presence of a slight excess of Na[BArF4] (typically 1.1 equiv) provides a more synthetically convenient means for generation of 2 in solution. In this way, dihydride complexes 4 and 5 were straightforwardly isolated in 64% and 74% yield, respectively, through reaction of 2 with 2,2′-bipyridine and IBioxMe4, respectively (Scheme 3). Both of these new complexes were characterized in the solid-state by X-ray diffraction, including the location of the hydride ligands in 4 from the Fourier difference map (Figure 1). The metrics about the metal center are notable for being in very good agreement with those of their

Scheme 4. Reactions of Dihydride Complex 2 with Ethylene and COE (L = IBioxMe4)a

a [BArF4]− anions omitted for clarity. All reactions in 1,2-C6H4F2 at 293 K; reactions with CO and C2H4 were carried out at 1 atm of pressure; reactions with COE used excess alkene (10 equiv/Ir).

former reaction with ethylene resulted in rapid and quantitative formation of the bis-ethylene complex trans-[Ir(IBioxMe4)2(C2H4)2][BArF4] (6) alongside a stoichiometric amount of ethane (observed) within 15 min, as indicated by 1H NMR spectroscopy. Under similar conditions, the addition of ethylene to the related but less sterically congested {Ir(PPh3)2(H)2}+ fragment has instead been shown to result in C


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Organometallics

Figure 2. Solid-state structures of 6 (left), 7 (center), and 8 (right). Thermal ellipsoids for selected atoms are drawn at the 50% probability level; most hydrogen atoms and anions are omitted for clarity. Selected bond lengths (Å) and angles (deg). 6: Ir1−Cnt(C2,C3), 2.048(3); Ir1− Cnt(C4,C5), 2.105(3); Ir1−C6, 2.071(3); Ir1−C21, 2.065(3); C2−C3, 1.387(5); C4−C5, 1.367(5); C6−Ir1−C21, 174.48(10); Cnt(C2,C3)−Ir1− Cnt(C4,C5), 173.17(13); ΘNHC(@C6), 173.8(3); ΘNHC(@C21), 178.2(3). 7: Ir1−Cnt(C2,C3), 2.066(2); Ir1−Cnt(C6,C7), 2.047(2); Ir1−C10, 2.088(2); Ir1−C25, 2.084(2); C2−C3, 1.398(3); C6−C7, 1.395(3); Cnt(C2,C3)−Ir1−Cnt(C6,C7), 85.28(9); C10−Ir1−C25, 100.07(8); ΘNHC(@ C10), 167.6(3); ΘNHC(@C25), 176.9(3). 8: Ir1−C2, 1.913(3); Ir1−C4, 1.899(3); Ir1−C6, 2.064(3); Ir1−C21, 2.064(2); C2−Ir1−C4, 170.13(14); C6−Ir1−C21, 177.15(11); all ΘNHC > 175.14

Figure 3. Optimized geometries of model isomers of 6, annotated with selected bond lengths (Å) and angles (deg)metrics associated with the alkene centroids in purple.21

the formation of a tris-ethylene adduct: trans-[Ir(PPh3)2(C2H4)3]+.15 The new complex was also prepared directly from 1 in 77% isolated yield. No significant reactions were detected by 1H NMR spectroscopy after 24 h when isolated 6 was dissolved in either CD2Cl2 or 1,2-C6H4F2 solution, consistent with good solution stability at ambient temperature (293 K). In CD2Cl2 solution, D2h symmetry is evident by 1H NMR spectroscopy across a wide temperature range (200−298 K, 500 MHz; and also by 13C NMR spectroscopy at 298 K, 126 MHz). In contrast, the X-ray structure (Figure 2) reveals an interesting and low symmetry arrangement of the ethylene ligands in the solid state; one ethylene ligand binds 75.3(2)° to the coordination plane (C2, C3) while the other adopts a more coplanar arrangement (C4, C5; 29.3(2)°).16 Associated with the approximate orthogonal geometry (|CCC−Cnt(CC)−Cnt(CC′)−CCC′|min =

75.8(4)°; Cnt = centroid), the perpendicular ethylene ligand has a shorter Ir−Cnt(CC) distance (2.048(3) vs 2.105(3) Å) and elongated CC bond length (1.387(5) vs 1.367(5) Å),17 suggesting stronger binding than the coplanar ligand. Although unstable with respect to ethylene dissociation in solution, a similar twisted bis-ethylene configuration has been observed in the disordered solid-state structure of a five coordinate Ir(I)− PCP pincer complex (|CCC−Cnt(CC)−Cnt(CC′)− CCC′|min = 39(2) [43%]/70(3) [57%]°).18 Other bis-ethylene iridium(I) complexes characterized by X-ray diffraction bear either cis-square-planar or cis-trigonal-bipyramidal geometries with parallel ethylene ligands.19 A search of the Cambridge Structural Database (v. 5.36) further emphasizes the peculiarity of this geometry, with only 8% of deposited transition-metal complexes containing two or more ethylene ligands, featuring D


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Organometallics

Consistent with this suggestion, 7 could also be prepared by transfer dehydrogenation on a similar time scale, employing instead 1, Na[BArF4], and excess COEand in this case only 1 equiv of COA was generated. A hydride species characterized by a 1H resonance at δ −29.26 is the major intermediate observed in both cases, which we tentatively assign to trans[Ir(IBioxMe4)2(COE)(H)2]+ (see Figure S13). Iridium complexes and in particular iridium pincer compounds are well documented to promote dehydrogenation reactions.22,23 Most closely related to the systems studied here is work reported by Crabtree et al., who described analogous formation of cis[Ir(PPh3)2(COD)]+ by transfer dehydrogenation of COE.24 The possibility of adapting this dehydrogenation reaction to a catalytic processes would be interesting; however, it is prohibited in this case by the strongly coordinated COD ligand (i.e. product inhibition). A feature of 7 quantified by very slow displacement of the diene under an atmosphere of CO at 293 K (t1/2 = 77 h) to afford trans-[Ir(IBioxMe4)2(CO)2][BAr F4 ] (8)the structure of which was verified by independent synthesis from 1 (Scheme 4, Figure 2). For comparison, and as expected, alkene substitution was an order of magnitude more rapid in 6 (t1/2 = 4 h).

similar ethylene geometries (i.e. within 20° of ideal orthogonal geometry).20 In order to further understand the structure of 6, a series of model conformational isomers bearing instead the less bulky IBioxH4 ligand were optimized employing density functional theory (DFT; BP86-D3/def2-TZVPP). These optimizations attempted to place the ethylene ligands in ideal orthogonal (6′), perpendicular (6a′), or coplanar (6b′) orientations Figure 3. In contrast to the distorted geometry observed experimentally for 6, the ethylene ligands in the optimized structure of 6′ bind with ideal angles (0°, 90°) with respect to the coordination plane. The distorted geometry was also retained on optimization of the full system (i.e. 6), suggesting the methyl substituents counteract adoption of an ideal orthogonal ethylene orientation (see Figure S14). Nevertheless, 6′ was found to be the lowest energy model conformer. Beginning the optimization with the alkene ligands perpendicular to the NHC−M−NHC vector led to the adoption of a complex with pseudo-saw-horse geometry, 6a′, 12.1 kJ mol−1 higher in energy than 6′, where the ethylene ligands are bowed away from linearity (Cnt(CC)−Ir−Cnt(CC′) = 145°). Restraining the ethylene ligands in an alternative parallel alignment also led to departure from square planar geometry and is much more destabilizing, with 6b′ calculated to be 80.4 kJ mol−1 higher in energy than 6′.21 The relative energies of these isomers lead us to hypothesize that the high symmetry of 6 observed in solution (200−298 K) is a result of time-averaged fluxionality originating from facile and synchronized rotation of the ethylene ligands about the respective metal−ligand vectorsthe high energy calculated for 6b′ is incommensurate with independent rotation. Energy decomposition analysis (EDA-NOCV) of the isomers indicated that the perpendicular ethylene ligand in 6′ shows a higher intrinsic bond strength than the coplanar ligand (ΔEint(C2H4) = −261.5 (perpendicular), −238.0 kJ mol−1 (coplanar); Table S1 and Figure S15). This is consistent with the experimental metrics and inspection of the deformation densities from NOCV analysis for the binding of the ethylene ligands in 6′ shows significantly larger absolute M → L and particularly L → M bonding interactions for the perpendicular ligand, summing up to an increased orbital interaction energy (ΔE orb = −518.2 kJ mol −1 (perpendicular), −446.1 kJ mol−1 (coplanar)). This energy gain is offset by a higher preparation energy for the perpendicular ligand in 6′ (|ΔΔEprep(perpendicular−coplanar)| = 23.8 kJ mol−1) and ultimately leads to essentially equal dissociation energies for the ethylene ligands (De(C2H4) = 167.4 (perpendicular), 167.6 kJ mol−1 (coplanar)). The ethylene dissociation energy is similar in 6a′ (De(C2H4) = 160.2 kJ mol−1); however, those in 6b′ are much less strongly bound (De(C2H4) = 87.7 kJ mol−1) due to lower intrinsic bond strength (ΔEint = −222.3 kJ mol−1) and higher preparation energy associated with the distorted metal fragment (Table S1). When mixtures of 3 and Na[BArF4] were reacted with an excess of COE (10 equiv) in 1,2-difluorobenzene at 293 K, an interesting transfer dehydrogenation reaction was observed by 1 H NMR spectroscopy leading to quantitative formation of a complex bearing η4-coordinated COD after 48 h, cis-[Ir(IBioxMe4)2(COD)][BArF4] (7), alongside sacrificial hydrogenation of 2 equiv of COE to COA (Scheme 4, Figure 2). When the reaction was performed on a preparative scale, 7 was obtained in 68% isolated yield. The stoichiometry of the reaction implicates initial generation of a reactive 12 VE fragment {Ir(IBioxMe4)2}+ through hydrogenation of COE.

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SUMMARY Following on from previous work employing the tetramethyl IBiox derivative (IBioxMe4), we have reported a method for accessing the formally 14 VE iridium(III) hydride fragment {Ir(IBioxMe4)2(H)2}+(2) in situ through simultaneous hydrogenation and halogen ion abstraction from trans-[Ir(IBioxMe4)2(COE)Cl] (1) in 1,2-difluorobenzene. The halide abstraction does not spontaneously proceed to completion, and instead dimeric [{Ir(IBioxMe4)2(H)2}2Cl][BArF4] (3) acts as the reservoir for 2 (in the presence of excess Na[BArF4]). Complex 3 was isolated and fully characterized in solution and the solid-state; both sets of data support the dimeric formulation, although a study of the isolated complex using variable temperature 1H NMR spectroscopy suggested that dynamic fragmentation of 3 occurs on the NMR time scale in solution. Consistent with this characteristic, stable dihydride complexes trans-[Ir(IBioxMe4)2(2,2′-bipyridine)(H)2][BArF4] (4) and [Ir(IBioxMe4)3(H)2][BArF4] (5) were readily obtained through in situ trapping of 2 using 2,2′-bipyridine and IBioxMe4. Building on these results, mixtures of 3 and Na[BArF4] were used as a latent source of 2 and reacted with excess ethylene and COE to afford trans-[Ir(IBioxMe4)2(C2H4)2][BArF4] (6) and cis-[Ir(IBioxMe4)2(COD)][BArF4] (7), respectively, through sacrificial hydrogenation of the alkenes; both are also accessible from halogen ion abstraction from 1 in the presence of the alkene. Complex 6 is notable for the unusual orthogonal arrangement that the trans-ethylene ligands adopt in the solid state, a conformation unique for d8 iridium complexes and very unusual for transition metal complexes generally. In solution, 6 is stable but highly fluxional on the NMR time scale (200−298 K, 500 MHz); a feature we attribute to facile synchronized rotation of the ethylene ligands on the basis of DFT calculations. The formation of 7 via transfer dehydrogenation of COE has literature precedent but importantly further reinforces the ability to partner IBioxMe4 with reactive metal centers capable of C−H bond activation, without intramolecular activation. Reaction of 7 with CO slowly formed trans-[Ir(IBioxMe4)2(CO)2][BArF4] (8), but the equivalent reaction E


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Organometallics

JHH = 1.3, 2H, C5,5′H{bipy}), 4.25 (s, 8H, OCH2), 1.30 (s, 24H, CH3), −21.33 (s, 2H, IrH, T1 = 701 ms). 13C{1H} NMR (CD2Cl2, 101 MHz): δ 162.3 (q, 1JBC = 50, ArF), 158.1 (s, NCN), 155.1 (s, C6,6′{bipy}), 138.6 (s, C2,2′{bipy}), 137.1 (s, C4,4′{bipy}), 135.4 (s, ArF), 129.4 (qq, 2JFC = 32, 3JBC = 3, ArF), 126.6 (s, C5,5′{bipy}), 125.4 (s, COCH2), 125.1 (q, 1JFC = 271, ArF), 123.9 (s, C3,3′{bipy}), 118.0 (sept, 3JFC = 4, ArF), 87.9 (s, OCH2), 63.7 (s, C(CH3)2), 25.3 (s, CH3). Anal. Calcd for C64H54BF24IrN6O4 (1630.16 g mol−1): C, 47.16; H, 3.34; N, 5.16. Found: C, 47.23; H, 3.21; N, 5.15. [Ir(IBioxMe 4 ) 3 (H) 2 ][BAr F 4 ] (5). To a mixture of trans-[Ir(IBioxMe4)2(COE)Cl] (0.0075 g, 0.010 mmol), Na[BArF4] (0.0097 g, 0.011 mmol), and IBioxMe4 (0.0023 g, 0.011 mmol) was added 1,2C6H4F2 (1 mL) under a H2 atmosphere (1 atm). The solution was stirred at room temperature for 16 h, then freeze−pump−thaw degassed three times within 6 h and placed under argon. The solvent was removed under a vacuum and the resulting the red solid extracted with dichloromethane (1 mL) to afford 5 after layering the solution with heptane (12 mL). Yield = 0.012 g (74%, red crystals). 1 H NMR (1,2-C6H4F2/C6D6, 400 MHz): δ 8.12−8.17 (m, 8H, ArF), 7.49 (br, 4H, ArF), 4.18−4.30 (m, 12H, OCH2), 1.63 (s, 6H, CH3), 1.62 (s, 6H, CH3), 1.59 (s, 6H, CH3), 1.33 (s, 6H, CH3), 1.06 (s, 6H, CH3), 1.04 (s, 6H, CH3), −28.47 (s, 2H, IrH). 1H NMR (CD2Cl2, 500 MHz): δ 7.69−7.74 (m, 8H, ArF), 7.56 (br, 4H, ArF), 4.49 (d, 2JHH = 8.4, 2H, OCH2), 4.44 (d, 2JHH = 8, 2H, OCH2), 4.41 (d, 2JHH = 8, 2H, OCH2), 4.40 (d, 2JHH = 8, 2H, OCH2), 4.37 (d, 2JHH = 8.4, 2H, OCH2), 4.36 (d, 2JHH = 8.4, 2H, OCH2), 1.69 (s, 6H, CH3), 1.66 (s, 6H, CH3), 1.62 (s, 6H, CH3), 1.42 (s, 6H, CH3), 1.15 (s, 6H, CH3), 1.11 (s, 6H, CH3), −28.57 (s, 2H, IrH, T1 = 940 ms). 13C{1H} NMR (CD2Cl2, 126 MHz): δ 166.3 ({t, 2JCH = 14}*, NCN), 162.3 (q, 1 JBC = 50, ArF), 151.5 ({t, 2JCH = 6}*, NCN), 135.4 (s, ArF), 129.4 (qq, 2 JFC = 32, 3JBC = 3, ArF), 127.2 (s, COCH2), 126.6 (s, COCH2), 125.7 (s, COCH2), 125.2 (q, 1JFC = 272, ArF), 118.0 (s, sept, 3JFC = 4, ArF), 88.4 (s, OCH2), 88.2 (s, OCH2), 87.5 (s, OCH2), 64.9 (s, C(CH3)2), 63.9 (s, C(CH3)2), 62.0 (s, C(CH3)2), 27.9 (s, CH3), 26.7 (s, CH3), 25.4 (s, CH3), 25.2 (s, CH3), 23.9 (s, CH3), 23.8 (s, CH3). * Resonances not fully decoupled in spectrum. Anal. Calcd for C65H62BF24IrN6O6 (1682.24 g mol−1): C, 46.41; H, 3.72; N, 5.00. Found: C, 46.34; H, 3.58; N, 5.13. trans-[Ir(IBioxMe4)2(C2H4)2][BArF4] (6). To a mixture of trans[Ir(IBioxMe4)2(COE)Cl] (0.025 g, 0.033 mmol) and Na[BArF4] (0.032 g, 0.037 mmol) was added 1,2-C6H4F2 (2 mL) under an ethylene atmosphere (1 atm). The yellow-orange solution was stirred at room temperature for 1 h, subsequently diluted with heptane (0.5 mL), and then filtered. Layering the filtrate with heptane afforded the orange crystalline product on diffusion. Yield = 0.039 g (77%, orange crystals). 1 H NMR (1,2-C6H4F2/C6D6, 400 MHz): δ 8.11−8.16 (m, 8H, ArF), 7.49 (br, 4H, ArF), 4.24 (s, 8H, OCH2), 3.11 (s, 8H, C2H4), 1.53 (s, 24H, CH3). 1H NMR (CD2Cl2, 500 MHz): δ 7.69−7.75 (m, 8H, ArF), 7.56 (br, 4H, ArF), 4.50 (s, 8H, OCH2), 3.20 (s, 8H, C2H4), 1.68 (s, 24H, CH3). 13C{1H} NMR (CD2Cl2, 126 MHz): δ 162.3 (q, 1JBC = 50, ArF), 145.9 (s, NCN), 135.4 (s, ArF), 129.4 (qq, 2JFC = 32, 3JBC = 3, ArF), 127.2 (s, COCH2), 125.2 (q, 1JFC = 272, ArF), 118.0 (sept, 3JFC = 4, ArF), 88.2 (s, OCH2), 63.8 (s, C(CH3)2), 59.0 (s, C2H4), 25.8 (s, CH3). Anal. Calcd for C58H52BF24IrN4O4 (1528.07 g mol−1): C, 45.59; H, 3.43; N, 3.67. Found: C, 45.48; H, 3.39; N, 3.74. cis-[Ir(IBioxMe4)2(COD)][BArF4] (7). To a mixture of trans-[Ir(IBioxMe4)2(COE)Cl] (0.075 g, 0.099 mmol) and Na[BArF4] (0.097 g, 0.109 mmol) was added 1,2-C6H4F2 (4 mL) under a H2 atmosphere (1 atm). The orange solution was placed under argon, and cis-cyclooctene (0.130 mL, 0.994 mmol) was added. The solution was stirred at room temperature for 48 h. After diluting the bright orange solution with heptane, the solution was filtered and layered with heptane to afford the crude product on diffusion, which was then recrystallized from CH2Cl2−heptane. Yield = 0.106 g (68%, orange crystals). 1 H NMR (1,2-C6H4F2/C6D6, 400 MHz): δ 8.11−8.15 (m, 8H, F Ar ), 7.50 (br, 4H, ArF), 4.23 (d, 2JHH = 8.4, 4H, OCH2), 4.18 (d, 2JHH = 8.4, 4H, OCH2), 3.98−4.01 (m, 4H, CH{COD}), 1.99−2.09 (m, 4H, CH2{COD}), 1.69 (s, 12H, CH3), 1.60 (app q, J = 8, 4H, 4

with bis-ethylene complex 6 was an order of magnitude faster, quantifying the strong coordination of COD in 7.


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EXPERIMENTAL SECTION

General Experimental Methods. All manipulations were performed under an atmosphere of argon, using Schlenk and glovebox techniques. Glassware was oven-dried at 150 °C overnight and flamed under a vacuum prior to use. Anhydrous CH2Cl2 and heptane (

Iridium Complexes of the Conformationally Rigid IBioxMe4 Ligand

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